The present invention relates to an electric servo assist steering system.
It is known to electrically operate power steering apparatus for assisting the driver of a vehicle to turn a steering wheel.
In the known art of electric power assist, the steering torque of a steering shaft coupled to the steering wheel is detected and an electric motor is energized based on the detected steering torque. The rotational power of the electric motor is transmitted through a ball-and-nut mechanism to the rack shaft to assist in turning the road wheels.
In the electrically operated power steering apparatus disclosed in Japanese laid-open patent publication No 59-50864, a ball-and-nut mechanism is positioned substantially centrally on a rack shaft, and the rack shaft is radially supported at five locations including bushings. It is difficult to keep the rack shaft in full coaxial alignment at all of the five supporting locations. Because of alignment errors of the rack shaft, different dimensional and assembling errors of the parts, and flexing displacements of the rack shaft caused by loads applied through tires and tie rods, the ball-and-nut mechanism and a rack-and-pinion mechanism are subject to undue forces tending to displace or deform them or tend to suffer undesirable friction.
When this happens, the driver of the vehicle experiences poor steering feel and the efficiency of the electric motor of the power steering apparatus is lowered.
An electric power assisted steering system is disclosed in Japanese laid-open utility model publication No 59-172072, wherein a rack shaft is radially supported at two locations, i.e., a ball-and-nut mechanism and a rack-and-pinion mechanism. Because the rack shaft is radially supported at two positions and is maintained in better coaxial alignment at those supporting positions, it is subjected to less undue forces. However, the rotor of an electric motor of the electrically operated power steering apparatus is supported in a cantilevered fashion due to its structural limitations. As a result, the distal end of the rotor is liable to interfere with an iron core that is fixed to rake housing.
In the electrically operated power steering apparatus disclosed in U.S. Pat. No. 5,650,701 an electric motor is disposed coaxially around the rack shaft in a housing, the electric motor comprising a stator fixedly mounted in the housing and hollow rotor rotary supported in the housing by a pair of bearings mounted on respective opposite ends of the rotor, the ball-and-nut mechanism being coupled to the rotor, rack shaft axially passing through motor rotor. This arrangement results in a large diameter housing that creates problem for installation in vehicles, particularly smaller vehicles where space is at a premium. Motors integrated into a housing have specific and tailored part designs that require special production for very different car models thereby increasing cost. Vehicle serviceability is also more expensive because any failure in the motor requires replacement of the entire assembly. Further, the motor shaft is directly connected to the ball nut. Motor torque is transmitted to axial movement of rackxe2x80x94thereby requiring a large motor, large electric voltage for power, and larger dimensions of electric cable to generate the axial forces. Their design has large inertial masses that have a negative effect on vehicle steering.
Disclosed is an electrically operated power steering apparatus that has an electric motor disposed parallel to a rack shaft. The system is designed such that the motor can be installed in any angle around a rack housing of small diameter. This permits easy installation and flexibility of design. The apparatus is also designed to accommodate off-the-shelf electric motors that are already commercially available so as to eliminate the need to customize an electric motor for each application.
The transfer of rotational energy from motor shaft to ball nut (secondary reducer) is realized by a primary reducerxe2x80x94a belt, silent chain, or gearing transmission. This primary reduction transmission will effect a large torque on the ball-nut and a larger axial force on the rack shaft that will satisfy the needs of even large vehicles. A smaller motor will be required, thereby also reducing the inertial mass on the rack.
There is no need for large motors, high voltages or amperages, or large diameters of electric cable for large axial steering forces requested by large vehicle.
Less expensive vehicle serviceability is achieved because a defective motor may be replaced without having to replace the entire steering apparatus.
A ball nut is installed between two angular contact ball bearings, which permit small angular rotations of the ball nut under steering forces. This reduces friction and insures smooth function of the ball-and-nut mechanism and the rack-and-pinion mechanism. Finally, the driver of the vehicle has a fine, smooth, quiet and comfortable steering feel, even driving large vehicles.
The motor-shaft and rotor are not directly fixed on the ball nut and adaptive ball-nut movements therefore do not disturb motor function.
The rack and pinion will also preferably have a xe2x80x9cVxe2x80x9d shape and needle bearing, which has the effect of preventing rotation of the rack and reduces the friction associated with movement.
The housing may be covered by a strong one-piece ferrous cover. This cover rigidly holds the correct distance between two angular contact bearings even at high temperatures. Belt, gearing, or silent chain transmission is installed inside the housing cover.
The steering wheel rotation is transmitted to the upper portion of the pinion shaft assembly. The pinion upper and lower shafts are connected by a torsion bar. Relative rotation of the upper pinion shaft relative to the lower pinion shaft is detected by torque sensor. This information is transmitted to an Electronic Control Unit that powers the electric motor. Rotation of electric motor is transmitted to axial movement of rack shaft by belt transmission and ball-and-nut mechanism. This steering system uses energy only when it works, saving energy in comparison to traditional hydraulic power systems, which use hydraulic pumps that consume energy even when the vehicle is stationary.
The power steering system of this invention has a lower negative impact on environment relative to actual hydraulic power system because of the cleaner exhaust and lack of hydraulic fluids.